Like the LGN, the striate cortex contains a retinotopic map. The representation of the macula occupies the most posterior and largest part of both gyri, and progressively more peripheral areas of the retina are projected to more anterior parts of these gyri. Overall, there is an orderly mapping of retinal loci across the surface of the striate cortex (see
Fig. 8.10
The geniculostriate pathway ends chiefly in layer 4 of the striate cortex (
Fig. 8.11 ), whereas the projection from the intralaminar LGN terminates in so-called blobs in layers 2 and 3. Similarly, axons that represent one eye or the other terminate within layer 4C in alternate adjacent patches that define ocular dominance columns. Cortical neurons in such a column respond preferentially to input from one eye. Near the border between two ocular dominance columns, neurons respond about equally to input from the two eyes.
The receptive fields of neurons in the striate cortex, aside from the monocular cells in layer 4C, are more complex than those of LGN neurons. Neurons in other layers may be binocular and respond to stimulation of both eyes, although the input from one eye often dominates (see ). In addition, cortical neurons outside layer 4C often show orientation selectivity (i.e., they respond best when the stimulus, such as a bar or an edge, is oriented and positioned in a particular way;
Fig. 8.12 ). These “simple cells” appear to be responding as though they received input from cells whose concentric center-surround receptive fields were arranged in such a way that their “on” centers were aligned in a row flanked by antagonistic regions. “Complex” cortical •Fig. 8.10 Theleftvisualfieldisrelayed(viatheLGNandvisualradiation)totheprimaryvisualcortexoftherighthemisphere,asapoint-to-pointretinotopicmap.Therepresentationofeachpartofvisualspaceisproportionaltothenumberofafferentaxonswithreceptivefieldsinthatpartofspace.Asaresult,theareaofmacularrepresentation(neartheoccipitalpole)islargerthanthatfortherestofthebinocularandmonocularfields.Notethatthelowerhalfofthefieldisrepresentedinthecuneusgyrusabovethecalcarinesulcusandtheupperhalfofthefieldinthelingualgyrusbelowthesulcus.(RedrawnfromPurvesD,etal[eds].Neuroscience. 3rded.Sunderland,MA:Sinauer;2004.) •Fig. 8.11 DiagramofvisualinformationflowintothevisualcortexfromtheLGNanditsprojectiontotheextrastriatecortex,tothesuperiorcolliculus(SC),andbacktotheLGN.M,magnocellularpath;P,parvocellularpath.(RedrawnfromSquireLR,etal[eds].Fundamental Neuroscience. SanDiego,CA:AcademicPress;2002.)LGN m cell LGN p cell LGN intralaminar p cell Cortex Cortex SC LGN 4B 2+3 5 6 4C˜4C°Blobs (2+3) neurons are similar to “simple” cells in that they are orientation specific, but instead of having flanking excitatory and inhibitory zones, they respond best to a particular stimulus orientation anywhere in their receptive field. They may also display direction selectivity; that is, they may respond when the stimulus is moved in one direction but not when it is moved in the opposite direction (see
Fig. 8.12 ). The receptive field of a “complex” cell may be thought of as a composite of adjacent “simple” cells with similar orientation selectivity. Because such neurons in a particular zone of the cortex all tend to have the same orientation selectivity, they are considered to form an orientation column (Fig. 8.13